The present invention relates to a process for reducing the concentration of hydrogen chloride in an aqueous suspension of a macromolecular chlorinated hydrocarbon containing said hydrogen chloride.
Macromolecular hydrocarbons such as polyolefins are generally chlorinated while in the form of finely-divided particles. One process provides for chlorination of the particles while they are suspended in an aqueous medium. Polyolefin resins may also be chlorinated while partially or totally dissolved in a solvent system. These processes generate a considerable amount of hydrogen chloride. The hydrogen chloride is dissolved in the aqueous medium or solvent system and distributed within the particles and on the surface of the particles. The acid must be removed or neutralized if the chlorinated polymer is to be useful for thermoplastic and elastomer applications. Several methods have been suggested for removing residual hydrogen chloride from macromolecular chlorinated hydrocarbons. One method relates to batch-wise centrifugation or filtration of the aqueous suspension the macromolecular chlorinated hydrocarbon and thorough batch-wise washing of the solid particles. However, a very large amount of water is necessary for reducing the amount of hydrogen chloride within and on the surface of the particles to an acceptable level. Since hydrogen chloride is not only present in the aqueous phase of the suspension but also within and on the surface of the macromolecular chlorinated hydrocarbon, reduction of the amount of hydrogen chloride to an acceptable level has proven to be very difficult. The macromolecular chlorinated hydrocarbons, such as chlorinated polyethylene, have relatively strong bonds to hydrogen chloride, due to their chloride content.
British Patent Specification No. 1,069,189 suggests a dialysis process for separating a strong acid of low molecular weight from an aqueous solution or suspension of an acidic polymer having a higher molecular weight, such as a sulfonated vinylaromatic polymer. An anion-exchange membrane is used for carrying out the dialysis process. The separation of the low molecular weight strong acid from the acidic polymer is based on the relative mobility and diffusion rate. The dialysis unit is designed to pass the dialysis feed liquor into one compartment and rinse water into the two adjacent ones so that the membrane is in contact on one side with dialysis liquor and on the other with water. For rapid removal of the strong acid from the feed liquor, the flow ratio of rinse water to dialysis feed should be high, preferably from 10 to 20:1. However, the use of large amounts of rinse water and the subsequent disposal of the rinse water containing the diluted strong acid are undesirable. Furthermore, according to the examples of the British patent specification only between 57 and 85 percent of the initial amount of strong acid in the feed liquor are removed.
According to another method, hydrochloric acid is neutralized by adding caustic soda or some other alkaline material. However, neutralization leads to the formation of chloride salts within and on the surface of the polymer particles. German Offenlegungsschrift DE-A-1 720 788 suggests treatment of an aqueous suspension of a macromolecular chlorinated hydrocarbon with ethylene oxide and/or propylene oxide in the presence of an alkaline material for neutralizing hydrochloric acid. Again, a chloride salt is formed within and on the surface of the polymer particles. The presence of such chloride salts results in poor electrical properties and high water swell of macromolecular chlorinated hydrocarbons.
U.S. Pat. No. 4,730,035 relates to a method of removing hydrogen chloride from moist chlorinated polyolefins by drying and stripping residual hydrogen chloride from the chlorinated polyolefins with a heated inert drying gas such as nitrogen, helium or argon.
It would be desirable if there were available a process for reducing the concentration of hydrogen chloride in an aqueous suspension of a macromolecular chlorinated hydrocarbon containing said hydrogen chloride which did not require neutralization of hydrogen chloride with an alkaline material and which did not require stripping of hydrogen chloride with a heated inert drying gas.